It is necessary to insure that the breathing passageways of certain medical patients, e.g., those in surgery or intensive care, are kept oPen at all times. This is accomplished in the prior art by means of an endotracheal tube which is inserted through the patient's nose or mouth and extends through the patient's throat and into the patient's windpipe or trachea. These prior art tubes are hollow and open at both ends, and the end that extends outside the mouth or nose is anchored in place, usually with tape. Air can then pass through the tube into and out of the patient's lungs.
The principal drawback of the prior art tubes is that the distal end of the tube inside the patient must be inserted to and kept at a relatively specific position which is at about the midpoint of the trachea. This is because if the tube is inserted too far into the trachea, its distal end may extend into the bronchial tree for one lung, and thus the other lung will receive no air and collapse. On the other hand, if the distal end of the tube is not inserted far enough, it may interfere with the vocal cords. Another related problem is that as the tube is being inserted, the distal end may enter the esophagus, which is behind the trachea, and no air may reach the lungs.
In a normal adult the trachea is about 11 centimeters in length, and the distal end of the tube is generally properly positioned at the trachea's approximate midpoint. It may be anchored in place there by expanding a balloon attached to the tube. This positioning, however, has much less margin for error in children or infants, whose tracheas are much shorter in length. Furthermore, for both adults and children, even if the tube is properly positioned initially, movement of the patient often causes the tube to move up or down, and therefore the location of the distal end of the tube must not only be positioned properly to begin with, but it must also be continuously monitored.
The prior art used several methods for determining the position of a tube inside a patent. First, the tube position can be determined by x-ray, but notwithstanding the possible adverse effect of continued exposure to x-rays, the principal drawback of this method is that by the time the x-ray is taken, developed and returned, the tube may have moved again. Accordingly, the most common real time monitoring methods involve listening to the chest to hear if both lungs are filling and visually observing of the depth markings on the exposed tube. Neither of these methods, however, is very precise.
Two earlier patents, Bresler U.S. Pat. No. 4,416,289 and Bresler U.S. Pat. No. 4,445,501, both assigned to the same assignee as this application and both incorporated herein by reference, disclose devices for determining the position of an endotracheal tube inside a patient in real time. In both, a narrow electromagnetic field is generated, which is disturbed when a band of ferromagnetic material on the distal end of an endotracheal tube enters it. As the field penetrates tissue without effect, the band disrupts the field when the endotracheal tube is positioned in the patient's throat. Since the field is small and narrow, the band (and hence the distal end of the tube to which it is attached) can be accurately detected and monitored in real time. This is far superior to the prior art methods.
As significant an advance as these devices are, however, there are two additional areas in which operation could be improved. First, in addition to detection per se, there should be a specific and predictable correlation from instrument to instrument between the response or triggering of the device and the distance between the device and the metal band. Specifically, the device should not be triggered even if it is directly above the band when the band is beyond a certain very short distance from the device. The reason is to avoid detecting the tube if it is in the esophagus. As the esophagus is located behind the trachea for a supine patient, absent a specific (and limited) uniform depth of detection for the devices, a tube could be detected as being properly in place when in fact it is actually in the esophagus behind the trachea. As a result, it is desirable to have an instrument that will detect the tube in the trachea but not when it is in the esophagus immediately behind it. This should be true for each instrument and for each size tube (Pediatric or adult).
Secondly, it is important that the device be relatively insensitive to changes in temperature and other such conditions. In some situations, temperature changes may cause a physical expansion or contraction of the windings used to generate the field. If this causes the field to become unbalanced, a false detection signal may result. Because of this, it is not advisable to use these devices under certain conditions, such as emergency treatment outdoors, because of the possibility that a false detection signal due to temperature change could be interpreted as an actual detection signal.
Accordingly, a general object of the invention is to provide a device for aiding in initially positioning and later monitoring an endotracheal tube in a patient's trachea in real time.
A specific object of the invention is to provide such devices which uniformly have a limited depth of detection so as to avoid detection signals when there is esophageal intubation.
Another specific object of the invention is to provide such a device in which the field windings and overall circuit are not sensitive to temperature or other such changes that would otherwise unbalance the field or produce a circuit change that would result in a false detection signal.